1. Field of the Invention
This invention relates to a transparent protective overcoat for a front-surface silver reflector. More particularly, the invention relates to a noble-metal overcoat and processes for its preparation.
2. Description of the Prior Art
Coating glass with silver to form a mirror is not new. However, since silver that is exposed to the atmosphere rapidly tarnishes due to chemical attack, it is necessary either to provide a protective coating over the silver or to otherwise prevent its being exposed to the atmosphere. Consequently, silver is used almost exclusively as a "back-surface" coating (for reflecting light that has passed through a transparent substrate), rather than as a "front-surface" coating. Even back-surface silver coatings require a protective overcoat. Protective coatings of metals, including gold, over back-surface silver coatings on mirrors were disclosed in U.S. Pat. No. 33,721, which issued Nov. 12, 1861, to J. von Liebig.
Front-surface silver mirrors have been prepared with protective layers of transparent dielectrics, such as MgF.sub.2 or oxides of aluminum and silicon. (See, e.g., G. Hass, J. Opt. Soc. Am. 72, 27 (1982)). Although these protective layers can actually enhance the reflectivity of silver, they have a tendency to degrade at high power levels of optical or other radiation and are subject to chemical attack, particularly at elevated temperatures. These limitations are particularly troublesome in laser applications, such as reflective coatings on the pump cavity walls of optically pumped lasers.